The present invention relates to photocatalytic units and, more particularly, to solid supports fabricated with same. The present invention also relates to an optoelectronic device incorporating the photocatalytic units and method of fabricating the same.
Nanoscience is the science of small particles of materials and is one of the most important research frontiers in modern technology. These small particles are of interest from a fundamental point of view since they enable construction of materials and structures of well-defined properties. With the ability to precisely control material properties arise new opportunities for technological and commercial development, and applications of nanoparticles have been shown or proposed in areas as diverse as micro- and nanoelectronics, nanofluidics, coatings and paints and biotechnology.
It is well established that future development of microelectronics, magnetic recording devices and chemical sensors will be achieved by increasing the packing density of device components. Traditionally, microscopic devices have been formed from larger objects, but as these products get smaller, below the micron level, this process becomes increasingly difficult. It is therefore appreciated that the opposite approach is to be employed, essentially, the building of microscopic devices from a molecular level up, primarily via objects of nanometric dimensions.
Solar cells or photovoltaic cells (PVC) are optoelectronic devices in which an incident photonic energy such as sunlight is converted to electrical power. The use of PVC are as alternative source for renewable energy gain importance because of the increasing cost of fossil oil, the adverse effect of pollution on the health and on the environment and the prospect of future depletion of the oil reserves. Current technology uses silicon-based or other types of semiconductor PVCs. PVC are already commercially available and most widely used with an average energy conversion efficiency of 13%. Under research and development are crystalline and thin layer silicon, GaAs and multi-junction devices some of which can reach up to 30% efficiency. Some of the high efficiency PVC are equipped with concentrating mirrors to reduce the size and therefore the cost of the PVC. A construction of an optimal PVC cell in which the efficiency per cost ratio is high is yet to be achieved. For this reason, various types of photo-active materials have been investigated in addition to Si and GaAs. Several inorganic materials such as CuInSe2, CdTe/Se, organic and dye synthesized molecules and polymeric films were investigated. Chlorophyll and chlorophyll derivatives were successfully used as sensitizing dyes in PVC [Radziemska, E. Progress in Energy and Combustion Science 2003, 29, 407-424] over the years. These materials are also useful for constructing light emitting devices.
Conventionally, these types of solar cells are fabricated by sandwiching a semiconductor p-n junction between a light transmissive electrode and an additional electrode. When a photon enters into the p-n junction, under an appropriate bias voltage, an electron-hole separation takes place and a photocurrent is generated. Presently known technology uses silicon-based or other types of PVCs. Such devices, however, are costly and their efficiency is far from being satisfactory. For example, commercially available silicon PVC is known to have an average energy conversion efficiency of 13%. It is expected that crystalline and thin layer silicon, GaAs and multi-junction devices which are currently under research and development, will reach efficiency of 24% for silicon and 34% for GaAs. These devices, however, are even more expensive than commercial silicon PVCs. To reduce cost, compromises are made on the size and bulkiness of the device. For example, known in the art are photovoltaic systems which incorporate mirrors to concentrate sunlight on small area of a photovoltaic cell.
Also known, are polymeric and dye-based PVCs. This technology, however, has not yet matured to provide high energy conversion efficiency. Polymeric and dye-based PVCs have reported to provide energy conversion efficiency of 5% or less.
Pigment-protein complexes which are responsible for photosynthetic conversion of light energy to chemical energy may be used as electronic components in a variety of light based devices. Although fabrication of molecular circuits is presently beyond the resolution of conventional patterning techniques such as electron beam lithography, positioning of molecules with sub nanometer precision is routine in nature, and crucial to the operation of biological complexes such as photosynthetic complexes.
Green plants, cyanobacteia and photosynthetic bacteria capture and utilize sunlight by means of molecular electronic complexes, reaction centers that are embedded in their membranes. In oxygenic plants and cyanobacteria, photon capture and conversion of light energy into chemical energy take place in specialized membranes called thylakoids. The thylkoids are located in chloroplast in higher plants or consists of foldings of the cytoplasmic membrane in cyanobacteria. The thylakoids, consisting of stacked membrane disks (called grana) and unstacked membrane disks (called stroma). The thylakoid membrane contains two key photosynthetic components, photosystem I and photosystem II, designated PS I and PS II, respectively. Photosynthesis requires PSII and PSI working in sequence, using water as the source of electrons and CO2 as the terminal electron acceptor.
PS I is a transmembrane multisubunit protein-chlorophyll complex that mediates vectorial light-induced electron transfer from plastocyanin or cytochrome C553 to ferredoxin. The nano-size dimension, an energy yield of approximately 58% and the quantum efficiency of almost 1 [K. Brettel, Biochim. Biophys. Acta 1997, 1318 322-373] makes the reaction center a promising unit for applications in molecular nano-electronics.
The crystalline structures of PS I from Synechococus elongatus and from plant chloroplast were resolved to 2.5 Å at 4.4 Å, respectively [P. Jordan, et al., Nature 2001, 411 909-917; A. Ben Shem, et al., Nature 2003, 426 630-635]. In cyanobacteria and plants, the complex consists of 12 polypeptides. Some of the polypeptides bind 96 light-harvesting chlorophyll and 22 beta carotenoide molecules. The electron transport chain contains P700, A0, A1, FX, FA and FB representing a chlorophyll a dimmer, a monomeric chlorophyll a, two phylloquinones and three [4Fe-4S] iron sulfur centers, respectively. The reaction center core complex is made up of the heterodimeric PsaA and PsaB subunits, containing the primary electron donor, P700, which undergoes light-induced charge separation and transfers an electron through the sequential carriers A0, A1 and FX. The final acceptors FA and FB are located on another subunit, PsaC. The redox potential of the primary donor P700 is +0.43 V and that of the final acceptor FB is −0.53 V producing redox difference of −1.0 V. The charge separation spans about 5 nm of the height of the protein representing the center to center distance between the primary donor (P700) and the final acceptor (FB). The protein complex is 9 nm in height and a diameter of 21 nm and 15 for the trimer and the monomer respectively.
It is recognized that in order to incorporate PS I reaction centers into molecular devices, it is essential to immobilize the PSI reaction centers onto a substrate without their denaturation.
In earlier works, care was taken to non-covalently attach plant PS I [I. Lee, et al, J. Phys. Chem. B 2000, 104 2439-2443; R. Das, Nano Letters 2004, 4 1079-1083] and bacterial reaction centers [C. Nakamura et al., Applied Biochemistry and Biotechnology 2000, 84-6 401-408; S. A. Trammell, et al., Biosensors & Bioelectronics 2004, 19 1649-1655] to solid surfaces so as to avoid inactivation of self-assembled monolayers.
Thus, genetic modifications of both a bacterial reaction center [S. A. Trammell, et al., Biosensors & Bioelectronics 2004, 19 1649-1655] and a plant PS I [Das, Nano Letters 2004, 4 1079-1083] by addition of a 6 histidine tail allows for non-covalent binding to a polymer coated metal surface. The histidine attached bacterial reaction center was shown to produce photocurrent in solution in electrochemical cell. The histidine tagged PS I was shown to be oriented but was not reported to produce either photocurrent or photopotential. In addition, the histidine tagged PS I as taught by Das supra, required stabilization using peptide surfactants in order to attach to solid surfaces. Lee et al., [J. Phys. Chem. B 2000] teaches coating a metal surface with organic molecules and adsorbing the PS I non-covalently to the organic layer. In this case, the proteins assumed several orientations. Lee et al., [Biosensors & Bioelectronics, 1996, 11-4, 375-387] teaches platinum precipitation on the surface of photosynthetic membranes, assuming formation of direct electrical contact with the acceptor side of PS I, because it can catalyze hydrogen evolution. Additionally, it has been shown that isolated PS I reaction centers can be platinized since after the platinization it produced hydrogen in the light.
However, none of these methods teach covalent attachment of functional PS I reaction centers to a solid surface and certainly not in an oriented manner. PS I reaction centers which are not oriented will cancel each other out, preventing the PS I-immobilized devices to be used in photoelectric devices such as solar batteries or logic gates.
There is thus a widely recognized need for, and it would be highly advantageous to synthesize active PS I reaction centers capable of binding to a solid surface in an oriented manner, thereby to allow fabrication of optoelectronic device devoid the above limitations.